1. Field of the Invention
The present invention relates to a system for the accurate measurement and control of image color values on a printing press with or without the presence of a color bar. More particularly, the invention provides a universal closed loop color control system and processes for controlling the color quality of color images printed on a substrate online or offline, with or without a color bar printed on the substrate.
2. Description of the Related Art
Color perception of a printed image by the human eye is determined by the light reflected from an object, such as a printed substrate. Changing the amount of ink or other medium applied to a substrate changes the amount of color on the printed substrate, and hence the quality of the perceived image.
Each of the individual single images is produced with a specific color ink, referred to in the art as “primary colors” or “process colors”. A multi-colored printed image is produced by combining a plurality of superimposed single color printed images onto a substrate. To create a multi-colored image, inks are applied at a predetermined pattern and thickness, or ink density. The ink patterns are generally not solid, but are composed of arrays of dots which appear as solid colors when viewed by the human eye at a distance. The images produced by such arrays of colored dots are called halftones. The fractional coverage of the dots of a halftone ink pattern combined with the solid ink density is referred to as the optical density of the ink pattern. For example, when ink dots are spaced so that half the area of an ink pattern is covered by ink and half is not, the coverage of the ink pattern is considered to be 50%.
The color quality of a multi-colored printed image is determined by the degree to which the colors of the image match the desired colors for the image, i.e. the colors of a reference image. Hence, the obtained quality of a multi-color image is determined by the density of each of the individual colored images of which the multi-colored image is composed. An inaccurate ink density setting for any of the colors may result in a multi-colored image of inferior color quality. An offset printing press includes an inking assembly for each color of ink used in the printing process. Each inking assembly includes an ink reservoir as well as a segmented blade disposed along the outer surface of an ink fountain roller. The amount of ink supplied to the roller train of the press and ultimately to a substrate, such as paper, is adjusted by changing the spacing between the edge of the blade segments and the outer surface of the ink fountain roller to change (either increase or decrease) the amount of ink printed onto the substrate in one or more ink zones (ink key zones). The position of each blade segment relative to the ink fountain roller is independently adjustable by movement of an ink control mechanism/device such as an adjusting screw, or ink key (ink control key), to thereby control the amount of ink fed to a corresponding longitudinal strip or ink zone of the substrate, wherein an “ink zone” (or “ink key zone”) refers to an area of the substrate extending across a width of the substrate. The ink control mechanism includes any device that controls the amount of ink fed to a corresponding longitudinal strip or zone of the substrate. The ink control keys each control the amount of ink supplied to a respective ink zone on the substrate.
In the printing industry, color bars have been used for a long time to measure ink density. A color bar comprises a series of color patches of different colors in each ink zone, wherein each color patch comprises one or more color layers. To achieve a desired (i.e. target) ink density for printed information on a substrate, the printing press operator measures the ink density of the color patch or patches in one or more ink zones. The ink density of a color is determined by the settings of the ink supply for the ink of that color. A printing press operator adjusts the amount of ink applied to the substrate to get a desired color having a desired ink density. Opening an ink key increases the amount of ink along its zone and vice versa. If the ink density of the patch is too low, the operator opens the ink key to increase amount of ink flowing to the substrate in the corresponding ink zone. If the ink density of the patch is too high, the operator closes the ink key to decrease the amount of ink flowing to the substrate. Generally, it is assumed that the change in color density of the patches also represents a similar change in the color density of the printed image. However, this assumption is not always correct. To adjust for this discrepancy, the press operator should take the color bar patch density only as a guide, while final color adjustments are made by visually inspecting the printed information, and also by measuring the color ink density, or color values, of critical areas in the print. Where used herein, the term “color” is used in reference to black ink, as well as inks of primary process colors cyan, magenta and yellow.
At the start of a printing run, the ink key settings for the various color inks must be set to achieve the appropriate ink density levels for the individual color images in order to produce multicolor images with the desired colors. Additionally, adjustments to the ink key settings may be required to compensate for deviations in the printing process of colors during a printing run. Such deviations may be caused by alignment changes between various rollers in the printing system, the paper stock, web tension, room temperature and humidity, among other factors. Adjustments may also be required to compensate for printing process deviations that occur from one printing run to another. In the past, such ink density adjustments have been performed by human operators based merely on conclusions drawn from the visual inspection of printed images. However, such manual control methods tended to be slow, relatively inaccurate, and labor intensive. The visual inspection techniques used in connection with manual ink key presetting and color control are inaccurate, expensive, and time-consuming. Further, since the required image colors are often halftones of ink combined with other ink colors, such techniques also require a high level of operator expertise.
Methods other than visual inspection of the printed image are also known for monitoring color quality once the press is running. Methods have been developed to control ink supplies based on objective measurements of the printed images. To conduct the task of color density measurement, offline density measurement instruments are available. Quality control of color printing processes can be achieved by measuring the optical density of a test target image. Optical density of various points of the test target image can be measured by using a densitometer or scanning densitometer either offline or online of the web printing process. Typically, optical density measurements are performed by illuminating the test target image with a light source and measuring the intensity of the light reflected from the image. For example, a press operator takes a sample of printed substrate with the color bars and puts it in the instrument. A typical instrument has a density scanning head traveling across the width of the color bars. After scanning, the instrument displays density measurements on a computer screen. Upon examining the density values on display and also examining the printed sample, the operator makes necessary changes to the ink keys. This procedure is repeated until satisfactory print quality is achieved.
To automate this task, online density measurement instruments are known. While the press is running, it is common for a press operator to continually monitor the printed output and to make appropriate ink key adjustments in order to achieve appropriate quality control of the color of the printed image. For example, if the color in a zone is too weak, the operator adjusts the corresponding ink key to allow more ink flow to that zone. If the color is too strong, the corresponding ink key is adjusted to decrease the ink flow. During operation of the printing press, further color adjustments may be necessary to compensate for changing press conditions, or to account for the personal preferences of the customer.
Online instruments comprise a scanning assembly mounted on the printing press. The test target image that is measured is often in the form of a color bar comprised of individual color patches. The color bar typically extends the width of the substrate (see FIG. 7). Typically, color bars are scanned on the printing press at the patches, which include solid patches and halftone patches for each of the primary ink colors, as well as solid overprints. The color bar is often printed in the trim area of the substrate and may be utilized for registration as well as color monitoring purposes. Each solid patch has a target density that the color control system attempts to maintain. The inking level is increased or decreased to reach this target density.
Instruments that can measure density on the press and also automatically activate ink keys on the press to bring color density to a desired value are commonly known as Closed Loop Color Controls. A Closed Loop Color Control is primarily used to perform three tasks. The first task is to analyze the image from pre-press information to find the coverage of different colors in different ink zones and preset the ink fountain key openings to get the printed substrate close to the required colors. Ink key opening presets are just an approximation and may not be a perfect setting. The second task is to analyze the color information scanned from the substrate being printed on the press, compare it with the desired color values and make corrections to the ink key openings to achieve the desired color values. The third task is to continuously analyze the printed substrate and maintain color values throughout the job run length.
Different density measuring instruments vary in the way they scan color bars and calculate color patch density. Different scanning methods can be categorized into two groups. A first group uses a spectrophotometer mounted in the imaging assembly. A video camera and strobe are used to freeze the image of moving substrate and accurately locate color bars. The spectrophotometer is then aligned to a color patch and it is used to take a reading of the color patch. For positioning color patches in the longitudinal Y direction of the substrate, a cue mark and a photo sensor are used. For distinguishing color patches from print, a special shape of color patch is required for this instrument. A second group uses video cameras mounted in an imaging assembly. Typically, a color camera with a strobe is used to freeze the motion of the moving substrate and acquire an image. Most manufacturers use a three sensor camera, in which prisms are used to split red, green and blue channels. Analog signals from these three channels are fed to frame acquiring electronics to digitize and analyze image.
Most manufacturers use xenon strobes for illuminating the moving substrate for a short period of time. Xenon strobes work on the principle of high voltage discharge through a glass tube filled with xenon gas. It is well known that the light intensity from flash to flash with such a device is not consistent. This becomes a problem in color measurement since variation in flash intensity provides false readings. To overcome this problem, a system described in U.S. Pat. No. 6,058,201 uses a light output measurement device in front of the strobe and provides correction in color density calculations. Another problem with xenon strobes is that they work with higher voltage and drive electronics generate electrical noise and heat. These features make it more difficult to package a camera and xenon strobe in a single sealed imaging assembly. Another prior system described in U.S. Pat. No. 5,992,318 mounts the strobe away from the camera and transmits light through a light pipe.
To overcome these problems, it is desirable to use white light emitting diode (LED) light strobes with a single sensor color camera to measure color values on the color bar to accomplish closed loop color operation on the press. White LEDs provide a light source with very consistent light from flash to flash. Also, the LEDs operate at a very low voltage and current. This reduces heat generation in the imaging assembly and it also eliminates electrical noise typically associated with xenon light strobes.
All of the above mentioned methods use a color bar with a combination of solid and tint patches to measure the color across the width of the substrate. Unfortunately, measuring the color of a printed substrate using a color bar has several disadvantages. First, it is an indirect method of measuring color in the print, whereby it is assumed that the change in color density of a patch in the color bar represents the change in the color value of the printed substrate in the longitudinal zone aligned with the measured patch. However, this assumption is not always correct. Second, the color bar requires additional space on the substrate. Depending on job configuration, this space may not be available. Further, this additional substrate space is not part of the finished product, so it increases the cost of production. In addition, there are associated trimming costs for printed products for which a color bar is objectionable, thereby increasing the cost of the operation, as well as the costs associated with removing and disposing of trimmed color bar waste.
Alternatively, measuring the color of a printed substrate with a color bar does have its advantages. First, a color bar provides dedicated patches for each color that can be measured by the control as well as by the press operators using hand held color measuring instruments. Further, different types of patches (such as 25% tint, 50% tint, 75% tint, trap overprint) can be printed to check overall performance including pre-press settings, ink and water balance.
For different press configurations and job requirements, it may or may not be possible to have color bars. While a color bar may have some advantages, the job and press configuration may not allow having a color bar. In such a case, the operator has to adjust the press by visually inspecting the image or by measuring the color value within the print using a hand held densitometer, and the operator has to choose the places where he would like to measure the color value, and the densitometer readings may not be correct if colors are mixed in the area being inspected. Due to the obstacles associated with color bars, it is desirable to provide an option to eliminate the color bar and automate the image inspection to significantly improve the overall efficiency of the printing process.
Several attempts have been made to measure color values in an image directly from a printed substrate. A number of past efforts have been explored through which color information on a print can be acquired and analyzed. For example, U.S. Pat. No. 5,967,050 teaches a method which takes images of a printed substrate and aligns the obtained image with a reference image from available pre-press information and calculates color error on pixel-by-pixel basis. The operation requires a lot of computation power making it very expensive and slow. These requirements make it practically impossible to implement Closed Loop Color Control without a color bar.
Another method of getting color information in each ink zone may involve taking multiple images in an ink zone and aligning and analyzing the images with the corresponding locations on the image information from the pre-press information on a pixel-by-pixel basis. This would also require a lot of computation power since images in the same ink zone have to be captured, aligned to the pre-press image, processed and analyzed.
Yet another method of getting the color information in each ink zone is by positioning a camera in an ink zone, illuminating the region under camera with a constant illumination light source (i.e. non-strobing) and keeping the camera shutter open for a certain time. In order to get a correct color reading, the shutter opening and closing should be synchronized with the substrate movement such that the number of press repeats passing under the camera are exact multiples, otherwise color information for the partial press repeat scanned is also added to the reading. Since color values read from the camera are dependent on the amount of light received by the sensor in a specific time, this method becomes speed sensitive. Any variation due to change in speed has to be compensated mathematically or by changing the light illumination intensity. Both solutions suffer from inherent inaccuracies and errors making it practically very difficult to implement this solution. This system is further disadvantageous because the light reflected from non-printed areas also gets integrated into the frame. If there is heavy coverage of various colors, the resulting integrated frame shows a very dark and gray looking frame. If there is a very small area being printed on the ink zone, the image of printed area gets diluted by the image of the non-printed area of the substrate to a point where the final frame may not be able to provide enough resolution information about the printed color.
A further method of obtaining color information in each ink zone is by keeping the camera shutter open for a time greater than the time for one press repeat to pass under the camera and using a strobe light to illuminate several sections of the ink zone and using the charge-coupled device (CCD) in the camera to accumulate the reflected color value for the whole repeat length. This method relies on the fact that the frame produced by such integration (multiple exposures) is a representative of total color in the ink zone area. The disadvantage of this system is that the light reflected from non-printed areas also gets integrated in the frame. If there is heavy coverage of various colors, the resulting integrated frame shows a very dark and gray looking frame. If there is a very small area being printed on the ink zone, the image of printed area gets diluted by the image of the non-printed area of the substrate to a point where the integrated frame may not be able to provide enough resolution information about the printed color.
The present invention provides an improved approach to measure color values on a printed substrate, where gray balance is monitored as well as overall color saturation in a printed image. The system of the present invention is capable of operation in either “Color Bar with Solid Ink Density” or “Gray Spot with Gray Balance” modes, where an operator has the choice to implement Closed Loop Color Control with or without a color bar printed on the substrate as per the methods of commonly owned U.S. Pat. Nos. 7,187,472 and 7,477,420, combined with the additional Gray Spot with Gray Balance feature of the present invention. More particularly, a Universal Closed Loop Color Control system is provided that allows real-time, four process color control and monitoring on a printing press using obscure gray dots printed in the page margins rather than color bars. The gray dots are unobtrusive, do not attract the eye and need not be trimmed, saving cost in labor and disposal. The system is universal by allowing the operator to choose and easily switch between the inventive gray spot (i.e. gray reference marker) analysis and conventional color bar analysis. The inventive system provides an alternative in the art for an efficient and inexpensive method for closed loop color control by allowing for measurement and determination of color density variations, as well as for controlling the plurality of ink control mechanisms, or ink keys, on a printing press for on-the-run color correction whether a color bar is present or not.
The process of the present invention is compatible with the operation of a printing press, such as sheet fed and web presses, and offset printing, Gravure printing, Flexo printing and generally any other printing processes. The system can communicate with the latest press controls as well as older presses for scanning, measuring and correcting color on the run.